An optical system of a writing head in an optical printer is usually designed so that an image of light spot of respective LEDs, (Light-Emitting Diode) constructing an LED array is focused on a photosensitive drum through a lens array. As a lens array, a distributed index rod lens array is often used.
A typical construction of an LED array, a distributed index rod lens array, and a photosensitive drum used in a conventional optical printer is shown in FIG. 1. In the figure, the LED is designated by reference numeral 10, the rod lens array by reference numeral 12, and the photosensitive drum by reference numeral 14.
An effective aperture angle θ of the lens array 12 is in the range of 17°-20° as a half angle, whereas the LED 10 emits light principally in Lambertian distribution, so that an optical availability efficiency is extremely low. The only 3-5% of the light emitted in a Lambertian distribution from the LED is transmitted to the photo-sensitive dram 14 through the lens array 12. This means that only the 95-97% of the light emitted by the LED is not utilized, resulting in a problem of a low optical availability efficiency.
In order to increase the optical availability efficiency, it is conceivable that a micro lens array is provided directly on the light-emitting area of the LED to cause the directivity of the light ray from the LED narrower so as to increase the light ray impinging into an aperture angle of the lens array. In general, the central part of the light-emitting area of the LED array used for an optical printer is covered by an electrode 20 extending into the light-emitting area 22, so that the shape of an effective light-emitting area 22 is approximately U-shaped as shown in FIG. 2. In order to cause the directivity of the light ray from the light-emitting area 22 narrower by means of a conventional micro lens array 18 as shown in FIG. 3, it is preferable that the light ray designated by a dotted-line 24 neighbored to an optical axis of the lens is utilized. However, the area neighbored to the optical axis corresponds to the position of the electrode 20, resulting in the problem such that the optical availability efficiency may not be fully increased.
An LED array provided with a micro lens array has been disclosed in Japanese Patent Publication Nos. 9-109455, 2000-347317, and 2001-36144. However, these publications have not discussed such a problem and the shape of a micro lens.
It is noted that the above-described problem is applied not only to an LED but also another light-emitting element.